US Patent 7092172 Imaging optical system and electronic apparatus using the same

The image forming optical system includes, in order from the object side, a meniscus, a first lens having positive refracting power with a convex surface directed toward the object side, an aperture stop, a meniscus, a second lens having negative refracting power with a concave surface directed toward the object side, a third lens having positive refracting power with a convex surface directed toward the image side, and a biconvex, fourth lens having positive refracting power. The fourth lens has at least one aspherical surface and satisfies the condition:1.0<φm/φp<50.0where φm and φp represent power of the fourth lens at a position of the maximum light height and at the paraxial position, respectively. Also, the following condition is satisfied:0.3<f1/f<2.0where f1 and f represent the focal lengths of the first lens and of the entire image forming optical system, respectively.